Fifth generation (5G) mobile networks are likely to be a combination of evolved third generation (3G) technologies, fourth generation (4G) technologies and emerging, or substantially new, components such as ultra-density network (UDN), also referred to as millimetre wave (mmW) radio access type (RAT).
Due to an increasing demand to enhance capacity in wireless networks and the lack of availability of spectrum in frequency ranges (e.g. 800 MHz-3 GHz), the use of frequencies in tens of GHz range is being investigated. Investigations are ongoing to explore high frequency bands, for instance, in the range of 30 GHz, 60 GHz and 98 GHz for the purpose of mobile networks. At these frequencies, a large bandwidth of spectrum is available. This means that both operating frequency and bandwidth for 5G networks are expected to be much higher than that used in legacy mobile networks, e.g. 3G and 4G networks. However, due to relatively large signal attenuation with respect to path loss, networks operating over such high frequencies are supposed to cover small areas with densely deployed radio access nodes (AN).
FIG. 1 illustrates one example of a mmW radio access network (RAN) structure. This mmW RAN example comprises one central control unit (CCU) 102, a group of access nodes (AN) 104A-D connected to the CCU, and two registered user equipment (UEs) 106A-B.
Such dense deployment of ANs is expected to provide sufficient coverage for indoor or hot areas (spots).
The CCU is responsible to configure the connected ANs. Currently it is supposed that the total carrier bandwidth of the mmW RAT may be up to 1 or 2 GHz. This bandwidth may be composed by a number of sub-band carriers of a certain bandwidth, e.g. 100 MHz.
It is expected that high gain beamforming is mandatory for mmW RAT. For example, for a mm-wave link at 60 GHz, a loss in received power caused by oxygen absorption may be as high as 16 dB/km from an AN. Moreover, the received power of a transmitted radio of a certain frequency is inversely proportional to said frequency squared. This means that, with the same propagation distance a wavelength of 60 GHz is attenuated 29.5 dB more than a wavelength of 2 GHz, even without considering the oxygen absorption.
When considering the oxygen absorption, high gain beamforming is expected to be mandatory in order to compensate the attenuation.
High frequencies correspond to small wavelengths. Thanks to relatively small wavelengths, more antenna elements can be integrated in an antenna panel of a given size, as compared to the number of antenna elements for larger wavelengths, i.e. lower frequencies. Using more antenna elements makes it possible to reach higher beamforming gain.
However, it is bad economy having one RF chain for each antenna element if there are several tens or several hundreds of antenna elements. In these cases, multiple antenna elements typically share one RF chain and analogue phase adjustment is applied for each antenna in order to adjust a beam direction to maximize a beamforming gain.
Further, broadcasted signals are advantageously transmitted in sweeping mode, in which broadcasted signals are transmitted in a number of beams forming a sweeping pattern.
Due to the large attenuation of mmW, see above, even broadcasted signals may be transmitted using beamforming.
Broadcasted signals may be divided into two types of broadcasted signals.
The first type may comprise pilot signals for cell searching, i.e. signals for synchronization of UEs to the broadcasting AN. The first type may also comprise minimum required mandatory system information.
Pilot signals may be transmitted in sweeping mode, for which each beam covers one sector and where there is partial overlap between neighbouring beams. As a result, a desired area may be covered by a set of beams.
FIG. 2 schematically illustrates one example of a beam sweeping pattern to provide omni-directional coverage applicable for transmitting a pilot signal by an access point (AP) or radio base station such as a eNB. Numbers 0-7 denote beams which together provide the omni-directional coverage.
In practice, a UE is supposed to blindly monitor the pilot beams in order to determine the strongest beam. The maximum number of blind detections, i.e. detection of the overall number of beams, may hence be proportional to the number of pilot beams, in this example.
The second type of broadcasted signals may comprise paging messages and broadcasted system information.
Further system information and some down link (DL) control messages, such as broadcast control messages, may also be broadcasted for the reason that the target receiver of these messages include UEs in idle mode and hence may await paging messages.
High gain beamforming may be also mandatory for the second type messages in order to provide large enough coverage. Further, it is desired to provide similar coverage for these second type messages as the one of the first type messages such as pilot signals for cell searching, for which reason sweeping transmission may also be applied for these second type messages.
During the sweeping interval of the send type messages, an idle UE is intended to decode these messages until they are detected or an interval during which the messages are swept ends.
It may be concluded that in the case of mmW RAT, the broadcasted signals may be transmitted in sweeping mode in order to achieve beamforming and meanwhile provide omni-directional coverage. However, from a client device perspective, such as a UE, which needs to monitor the broadcasted signals from a transmitting AN (or network node), it means a relatively large power consumption to monitor these broadcasted signals due to the client device may need to decode the transmitted signal many times until the transmitted signal is detected, or the sweeping interval ends. The power consumption is even higher for a client device that is in idle mode. Since such client device (or UE) also monitors the system information and paging messages.
There is hence a need for a solution addressing the issues as discussed above.